How the interaction occurs:When these two medicines are taken together, tetracycline may interfere with the beneficial effects of penicillin.

What might happen:A decrease in the beneficial effects of your penicillin antibiotic may occur, which would decrease the effectiveness in treating your infection.oxytetracycline im decreases effects of penicillin g sodium inj by opposing drug effects.

Patient Drug Interactions Source: RxList

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Thank you very much,

What would be the possible alternative, when you must give penicillin intra-abdominal or im and at the same time the uterus is infected (dystcocia,.....). 

Intrauterine antibiotics (Billy I. Smith and Carlos A. Risco,   2009): 

Penicillin administered systemically is a treatment option for endometritis, postpuerperal metritis, and toxic puerperal metritis. Penicillin is preferred to other antibiotics because its distribution to all layers of the uterus is excellent,1,39 it is inexpensive, and it has established milk and meat withholding times when used according to label. Unfortunately, treating postpartum metritis with penicillin is an off-label use because the drug is not indicated for the disease and the dosage of 9,000 to 18,000 U/lb IM once or twice daily is three to six times higher than what is recommended on the label. There is no known withholding time for using penicillin in an offlabel manner. However, having an available, easy-to-use residue test kit helps make penicillin a preferred treatment of postpartum metritis.

Oxytetracycline is also used to treat postpartum metritis. It is most often used in cows suffering from less severe forms of metritis (i.e., endometritis, postpuerperal metritis).8 There are many concerns about treating postpartum metritis with systemic oxytetracycline. Research by Bretzlaff et al 29 revealed that 11 mg/kg IV q12h rather than q24h was needed to pro-vide uterine tissue concentrations that would combat infection. This is an impractical regimen. Because of these concerns, the use of systemic oxytetracycline has not been considered the preferred choice for treating postpartum metritis.

Some authors have recommended systemic penicillin in conjunction with intrauterine oxytetracycline for treating toxic puerperal metritis.35,40 However, the combination of penicillins and tetracyclines has resulted in residues in milk for excessively long periods.41 This results in discarded milk for a prolonged period, thereby decreasing milk revenues. Furthermore, the failure of dairies to heed milk withholding times or perform milk antibiotic residue tests may lead to antibiotic contamination of milk.

A reliable treatment alternative that controls the effects of postpartum metritis and avoids the problem of milk antibiotic residues is needed. The use of ceftiofur sodium (which has a zero milk withdrawal time at the recommended label indication, dose, and route) for treating toxic puerperal metritis has been evaluated. It is important to note that ceftiofur sodium is not labeled for treating cows diagnosed with postpartum metritis. Therefore, its use is considered extralabel.

It is better to avoid any intrauterine medication as n number of systemic broad spectrum antibiotics are  available in the market and which will give you better results. 

Further some bacteriostatic drugs impair the action of bactericidal drugs. In this sense  it is better not to use tetracycline with penicillin.